Fix inconsistent apt usage and duplicate JSC package in Linux build instructions (#1627)

The Linux build instructions listed `libjavascriptcoregtk-4.1-dev` in
the base mandatory packages, causing it to be installed for all engine
targets (including V8) and duplicating the engine-specific install step.
The base command also used `apt` while engine-specific steps used
`apt-get`.

## Changes

- **Remove `libjavascriptcoregtk-4.1-dev` from base install** —
engine-specific packages now live only in their respective engine
sections
- **Standardize on `apt-get`** throughout the Linux section for
consistency

Before:
```
sudo apt install libgl1-mesa-dev x11proto-core-dev libx11-dev libcurl4-openssl-dev libjavascriptcoregtk-4.1-dev g++ cmake ninja-build npm
```

After:
```
sudo apt-get install libgl1-mesa-dev x11proto-core-dev libx11-dev libcurl4-openssl-dev g++ cmake ninja-build npm
```

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Author: Copilot

BUILDING.md

@@ -279,7 +279,7 @@ The minimal requirement target is an OpenGL 3.3 compatible GPU. Clang 9+ or GCC
 First step is to install packages mandatory for building. For example, with a GCC toolchain:
 
 ```
-sudo apt install libgl1-mesa-dev x11proto-core-dev libx11-dev libcurl4-openssl-dev libjavascriptcoregtk-4.1-dev g++ cmake ninja-build npm
+sudo apt-get install libgl1-mesa-dev x11proto-core-dev libx11-dev libcurl4-openssl-dev g++ cmake ninja-build npm
 ```
 
 Depending on the JavaScript engine you want to use, you will have to install the package accordingly: